Traditional clamp style upper blocks fit poorly and can exert enough inward pressure to damage the receiver when applying torque to a barrel nut or flash suppressor. That€™s bad design. So, we designed an upper block that leverages the strongest part of the gun € the barrel lugs inside the star chamber. The Lug-Lok is precision machined and precisely fits into all Mil-Spec uppers. It mates perfectly with the barrel lugs, while allowing you to rotate the upper to be able to work on different sides. There are 2 vise pins that allow you to hold the upper at multiple working angles and prevent unwanted rotation during high torque application. The lock knob keeps Lug-Lok seated into the barrel lugs. The result € the safest, easiest and smartest way to lock down an AR15 upper into a bench vise.